Numerous reports agree that through its interaction with insulin prolactin promotes mitosis in the mammary gland and that mitosis increases in the avian crop when prolactin is administered. On the other hand, published findings regarding the action of prolactin on cell division in the stratum corneum of the urodele have been contradictory. In an approach to the resolution of these contradictions, prolactin will be administered on alternate days to groups of intact newts and newts that had been previously hypophysectomized. Tritiated thymidine will be injected into the experimental and into intact and hypophysectomized control groups. Mitotic figures will be counted and thymidine uptake was measured in the epidermis. Results from these two procedures should show whether or not there increase in cell division in the epidermis of the prolactin-injected animals over that of the control animals in both intact and hypophysectomized series. In the autumn the male red-spotted newt develops nuptual pads and the height of its tail fin increases to a maximal proportion. At the end of the spring breeding season the nuptual pads are lost and the tail fin is markedly reduced in height so that an annual cycle ensues. A similar loss of pads and tail fin height occurs when pad-bearing males are maintained under laboratory conditions. The hormonal control of these changes will be investigated by the administration to both intact and hypophysectomized animals of prolactin, testosterone and other hormones which earlier work or preliminary studies have shown to act on pads and/or tail fins.